Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel or power generation and as a fuel for domestic cooking. The process of obtaining natural gas from an earth formation containing typically including drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Therefore, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the cost of the natural gas and is not economical.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In the second transformation, known as the Fischer-Tropsch process, carbon monoxide (CO) reacts with hydrogen (H2) to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Hydrocarbons having carbons linked in a straight chain are known as aliphatic and could be saturated (paraffins) or unsaturated (olefins). Paraffins are particularly desirable as the basis of synthetic diesel fuel. In addition other products containing oxygen, hydrogen and carbon such as oxygenated hydrocarbons are formed.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts have the desirable function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table. The metal in the catalyst tends to facilitate reaction by forming temporary bonds with both carbon monoxide and hydrogen, thus bring carbon monoxide molecules into physical proximity with hydrogen molecules. The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then desorb from the catalyst and can be collected.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of weights. Thus, the products produced by a Fischer-Tropsch synthesis contain a range of hydrocarbons that can include gas, liquids and even waxes. Hydrocarbon waxes can be hydrocracked to produce lighter liquids and/or gases. For example, a method of producing diesel fuel may include distillation to separate wax fractions from lighter hydrocarbons, hydrocracking of the wax fraction, and further distillation of the hydrocracking products to separate the diesel fraction of the hydrocracking products.
Originally, Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as difficulty of temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors, sometimes called “slurry reactors” or “slurry bubble columns,” are well known in the art. In a gas-agitated multiphase reactors, catalytic particles are suspended in liquid and gas reactants are fed into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. The liquid products are recovered from the suspending liquid using any suitable technique, such as settling, filtration, magnetic separation, hydrocycloning, or the like, and then further separating the fluids.
Gas-agitated multiphase reactors or slurry bubble column reactors inherently have very high heat transfer rates; therefore, reduced reactor cost. The ability to remove and add catalyst online is one of the principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Appl. Catalysis A: General, (1999) 186 55-70) give a history of the development of various Fischer Tropsch reactors and the advantages of slurry bubble columns over fixed bed reactors Additionally it is well known that a Fischer-Tropsch reactor system can comprise a single stage or multiple stages, and can comprise a single reactor vessel or multiple reactor vessels per stage. In the case of a multistage reactor system, a stage may sometimes be defined as a “pass.” A per-pass conversion, for example, represents the conversion obtained after one stage.
Yates and Satterfield (Energy and Fuels, (1991) 5, 168-173), provide an equation that, when combined with the hydrodynamics and mass transfer predictions, can be used to evaluate the performance of slurry bed reactor using cobalt catalysts. In its full form, that equation is given by Equation (1).Rate (CO hydrogenation)=Ae−E/RT(PH2·PCO)/(1+a PCO)2  (1)where A is the intrinsic rate, E is the activation energy, R is the gas constant, T is the temperature (° K), and PH2 and PCO are the partial pressures of H2 and CO, respectively. In practical applications, when the partial pressure of CO is greater than about 0.5 Bar, Equation (1) can be simplified to Equation (2):Rate α PH2/PCO   (2)indicating that the rate of hydrogenation in a FT reactor is a function of the ratio of the concentration of H2 to the concentration of CO, sometimes hereinafter referred to as the H2:CO ratio.
Since the rate of conversion of H2 can be different from that of CO, the exit H2:CO ratio at the exit of the reactor where the reaction takes place can be lower or higher than the initial H2:CO ratio at the inlet of that reactor, depending on whether the initial H2:CO ratio is lower or higher than the H2:CO usage ratio. Marc Dry in Catalysis Today 71, (2002) 228-229 defines the usage ratio for cobalt-based FT catalysts to be 2.15, whereas for iron-based FT catalysts, the usage ratio varies depending on the water-gas shift reaction which converts some of the CO to CO2. The usage ratio for iron-based FT catalysts is typically lower than that of Co-based FT catalysts. As an example, the H2:CO usage ratio is about 1.7 for the low-temperature FT process. When the initial H2:CO ratio is lower or higher than the usage ratio, the conversion rate of H2 is greater or lower, respectively, than that of CO. FIG. 1 illustrates the change in the exit H2:CO ratio of a FT reactor with a cobalt-based catalyst for different conversions when the inlet H2:CO ratio is below (▴), at (-·-), and above (□) the usage ratio of 2.15.
Hence, when the initial H2:CO ratio to a Fisher-Tropsch reaction system is lower than the usage H2:CO ratio, then the H2:CO ratio will be even lower at the exit of the reactor, and consequently in the case of a multistage reactor system, the H2:CO ratio will decrease with each successive pass through each reactor. One the other hand, when the initial H2:CO ratio to a Fisher-Tropsch reaction system is higher than the usage H2:CO ratio, the H2:CO ratio will be even higher at the exit of the reactor, and consequently in the case of a multistage reactor system, the H2:CO ratio will increase with each successive pass through each reactor. The usage H2:CO ratio for any given catalyst is defined as the H2:CO ratio which remains unchanged throughout the reactor regardless of conversion rate, i.e., the H2:CO ratio in the feed to the reactor equals that of the exit of the reactor, due to identical conversion rate of H2 and CO.
Considerable patent literature addresses the optimization of the Fischer Tropsch Slurry Bubble Column reactor (SBCR) and the overall system. U.S. Pat. No. 5,348,982 shows one mode of operation for SBCR. U.S. Pat. No. 6,060,524 and U.S. Pat. No. 5,961,933 shows that an improved operation can be obtained by introduction of liquid recirculation. U.S. Pat. No. 4,754,092 discloses a process for reducing methane formation and increasing liquid yields in Fischer-Tropsch hydrocarbon synthesis processes comprising adding one or more olefins to the reactor bed at a point below 10% of the distance from the top to the bottom of the reactor bed and above a point 10% above the bottom of the reactor bed to the top of the reactor bed in an amount sufficient to reduce said methane formation.
Despite all the development to date, there remains a need for an optimized Fischer Tropsch reactor and reactor configuration. In particular, there are continuing efforts to design reactors that are more effective at producing products in the desired range. In some instances it is particularly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with five or more carbon atoms per hydrocarbon chain (C5+), and still more desirable to maximize the production of C9+ hydrocarbons. Components (C9+) that boil at temperatures above about 150° C., are herein defined as “heavy components” and are generally desirable, whereas C2-C9 are referred to herein as “light components.” Furthermore, light olefins, i.e. unsaturated hydrocarbons having 2 to 9 carbons, are typically not desired products.
It is not uncommon, therefore, to recycle light olefins to a Fischer Tropsch reactor, with the expectation that they will undergo further chain growth, as illustrated in FIG. 2A. Because the recycled olefins can also undergo hydrogenation and form the corresponding paraffins, however, as shown in FIG. 2B, it is not possible to ensure that the olefins will undergo the desired chain growth. Furthermore, because the Fischer-Tropsch reaction is inherently a hydrogenation reaction, it is particularly difficult to achieve the desired chain growth.
Hence, it is desirable to design a gas-agitated multiphase reactor system that enhances the productivity of a Fischer-Tropsch system by increasing the degree of chain growth and minimizing the degree of hydrogenation that occurs in a recycled olefin stream. It is believed that increasing chain growth will in turn result in improved overall reactor productivity of C9+ hydrocarbons and/or reduced reactor volume.